The present invention relates to a safety device for protecting a speed change gear system for a bicycle more particularly to a safety device which is associated with a gearshift wire and a tubular sheath therefor in order to prevent a rear derailleur, a speed change lever and/or the gearshift wire per se from being damaged by the careless operation of the speed change lever by a bicyclist.
In such type of bicycles equipped with a multiple free-wheel and a rear derailleur, the speed change can be effected by transferring the endless chain from one sprocket wheel to another only when the chain is in its running motion. If the speed change lever is operated in a careless attempt when the bicycle is standing still and the chain is not in running motion, the gearshift wire, the speed change lever and/or the rear derailleur may occasionally be damaged. To describe this in detail in conjunction with FIG. 7 of the accompanying drawings, in the state where a tubular sheath (S) having a curved posture is rigidly secured at its ends (P, Q) and the gearshift wire (W) is fixed at its end (E), and the curved part of the gearshift wire (W), which is disposed within the tubular sheath (S) so as to extend therethrough, is forcibly pulled in the direction of an arrow (A), the curved part of the gearshift wire is naturally strained to draw near rectilinearity. However, the tubular sheath (S) maintains its original curved posture because it is usually made of inelastic material and rigidly secured at its ends (P, Q). Consequently, the gearshift wire is hindered from approaching rectilinearity under the restrictive function of the curved tubular sheath holding the curved part of the wire (W) therein, resulting in that the gearshift wire undergoes such excessive tension that may cause the wire to be undesirably stretched or make parts of the rear derailleur and/or the speed change lever forcibly deformed.
In order to eliminate the above-mentioned deficiencies, an improved type of safety device has been proposed in U.S. Pat. No. 3,588,077 to Ozaki, in which one end (R) of the inelastic tubular sheath (S) is arranged so as to be axially movable and a coil spring (C) is disposed about the gearshift wire (W) and connected to said end (R) of said sheath (S). According to this structure of U.S. Pat. No. 3,588,077, when the gearshift wire (W) is pulled, in operation, in the direction of the arrow (A), the movable end (R) of the sheath (S) is shifted in the same direction, compressing the coil spring (C), so that the sheath may be permitted to deform into a more rectilinear posture, as the curved part of the gearshift wire (W) is forced to approach rectilinearity. Thus, the above-mentioned deficiencies may be overcome.
However, according to the above-described structure of the safety device of said U.S. patent to Ozaki, the coil spring (C) utilized therein must be long enough to permit movement through a sufficient distance of the end (R) of the sheath (S) in order to obtain the desirable deformation of the sheath. However, if such a long coil spring is employed, the spring constant thereof is inevitably reduced and the stiffness of spring is insufficient, which causes inferiority in the expected function of the gearshift wire. Further, by employment of a long coil spring, the axial dimension of the whole safety device becomes unfavorably larger.
Therefore, one object of the present invention is to provide an improved safety device for the speed change gear system of a bicycle, which may eliminate the above-mentioned deficiencies seen in the conventional safety device.
Another object of the invention is to provide a safety device for the speed change gear system of a bicycle, which is compact in size but permits movement through a sufficient distance of the movable end of the tubular sheath.
A further object of the invention is to provide a safety device for the speed change gear system of a bicycle, which utilizes a specific combination of at least one coil spring of a large diameter and a further coil spring of a small diameter for absorbing excessive tension in the gearshift wire.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof taken in conjunction with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as to define the limits of the present invention, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in art from the detailed description.